Reactivity of Dissolved Organic Matter with the Hydrated Electron: Implications for Treatment of Chemical Contaminants in Water with Advanced Reduction Processes

Advanced reduction processes (ARP) have garnered increasing attention for the treatment of recalcitrant chemical contaminants, most notably per- and polyfluoroalkyl substances (PFAS). However, the impact of dissolved organic matter (DOM) on the availability of the hydrated electron (eaq–), the key reactive species formed in ARP, is not completely understood. Using electron pulse radiolysis and transient absorption spectroscopy, we measured bimolecular reaction rates constant for eaq– reaction with eight aquatic and terrestrial humic substance and natural organic matter isolates ( kDOM,eaq–), with the resulting values ranging from (0.51 ± 0.01) to (2.11 ± 0.04) × 108 MC–1 s–1. kDOM,eaq– measurements at varying temperature, pH, and ionic strength indicate that activation energies for diverse DOM isolates are ≈18 kJ mol–1 and that kDOM,eaq– could be expected to vary by less than a factor of 1.5 between pH 5 and 9 or from an ionic strength of 0.02 to 0.12 M. kDOM,eaq– exhibited a significant, positive correlation to % carbonyl carbon for the isolates studied, but relationships to other DOM physicochemical properties were surprisingly more scattered. A 24 h UV/sulfite experiment employing chloroacetate as an eaq– probe revealed that continued eaq– exposure abates DOM chromophores and eaq– scavenging capacity over a several hour time scale. Overall, these results indicate that DOM is an important eaq– scavenger that will reduce the rate of target contaminant degradation in ARP. These impacts are likely greater in waste streams like membrane concentrates, spent ion exchange resins, or regeneration brines that have elevated DOM concentrations.

Absorbance measurements for specific ultraviolet absorbance at 254 nm (SUVA254, L mgC -1 m -1 ) and spectral slope (S300-600, nm -1 ) calculations were performed on a Cary-100 UV-vis spectrophotometer (Agilent).SUVA254 and S300-600 were then calculated using eq.S1 and S2, respectively, where  254 is the absorbance at 254 nm wavelength (cm -1 ), [DOC] is the concentration of dissolved organic carbon (mgC -1 L) calculated based on the measured mass of isolate and the % m/m carbon, aλ is the absorbance (cm -1 ) at wavelength  is the wavelength, and   is the reference wavelength (300 nm). 1 Spectral slope was calculated from eq.S2 using the exponential fitting function in Excel.Table S3 lists the SUVA254 and S300-600 values for each DOM isolate.

Text S2. Electron pulse radiolysis measurements.
The radiolysis of water initiated by a fast electron pulse produces several radical and molecular species as observed in eq.S3, 2 where the bracketed numbers are the G-value or yield (µM J -1 ) of species produced at 10 -7 s after irradiation.The transient decay kinetics of eaq -were monitored at 720 nm using transient absorption spectroscopy.No effort was undertaken to isolate eaq -by adding t-butanol because we were concerned that t-butanol could impact DOM macrostructures.Although the presence of other radical species like • OH can impact the eaq -lifetime in solution, this is not of concern here for two reasons.First, although • OH will react with eaq -, the rate will be the same in all solutions because the nominal pulse intensity is the same in each experiment and the rate is overall low due to the low concentration of each radical species (ca.2-4 µM for each radical).Thus, eaq -reaction with • OH will be a constant component of the background eaq -decay.Second, there will be significant scavenging of the • OH radical by the DOM itself, 3 which will further reduce the • OH radical free concentration.Also, it is unlikely that eaq -would react with a moiety in DOM oxidized by • OH in the same µs timescale.
Figure S1 shows first-order eaq -decay constants derived from transient absorption data plotted against DOM concentration.The slope of each line represents the  ,  − .The y-intercept represents any eaq -scavengers other than DOM present in the background anerobic water.For example, H + will be an important eaq -scavenger for experiments conducted at acidic pH given the high bimolecular rate constant of 2.3×10 10 M -1 s -1 . 4Given that the background solvent's scavenging capacity remains the same, the change in first order eaq -decay constants are determined exclusively by changes in the DOM concentration.H + in this instance serves as a constant background eaq -scavenger under all experimental conditions.

S8
Table S4.Bimolecular rate constants for DOM with different oxidizing radicals.

Text S4. MnQ calculations.
The number average molecular charge (MnQ) was calculated as a product of the number average molecular weight (Mn, Table S5) and the total charge density (Q, Table S6).Q was calculated by eq.S4, where Q1 and Q2 are the charge densities, K1 and K2 are the average equilibrium constants, and n1 and n2 are the empirical parameters from pH titration data. 9The % m/m refers to the conversion of the charge density from a per gram humic substance (gHS) to per gram carbon (gC) basis.

Figure S1 .
Figure S1.Kinetic data for DOM-eaq-bimolecular rate constant determination.Line represents a linear fit to the data using the least squares method with the slope reported as the bimolecular rate constant ( ,  − ).Aquatic DOM isolates (blue color) analyzed include A) SRNOM II, B) SRHA II, C) MRNOM, D) PLFA, and SRFA II (Figure 1C, main manuscript).Terrestrial DOM isolates (brown color) analyzed include E) ESHA IV, F) PPFA II, and G) PPHA I. Markers represent pseudo-first-order rate constants determined from transient eaq -decay data and error bars represent uncertainty of the fitted data (majority of error bars are within markers).Insets show data plotted on equivalent y-axis to compare between samples.Experiments conducted at pH 7.0 ± 0.1, 22 ± 2 ⁰C, and 10.0 mM dibasic phosphate buffer.

Figure S2 .
Figure S2.Kinetic data for DOM-eaq -bimolecular rate constant determination for varying IHSS catalog numbers.Line represents a linear fit to the data using the least squares method with the slope reported as the bimolecular rate constant ( ,  − ).Aquatic DOM isolates (blue color) analyzed include A) SRFA I, B) SRFA II, C) SRHA II, and D) SRHA III.Markers represent pseudo-first-order rate constants determined from transient eaq -decay data and error bars represent uncertainty of the fitted data (majority of error bars are within markers).Insets show data plotted on equivalent y-axis to compare between samples.Experiments conducted at pH 7.0 ± 0.1, 10.0 mM dibasic phosphate buffer, and controlled temperature (e.g, 25 ⁰C for A) and D), 20 ⁰C for B), and 22 ⁰C for C)).

Figure S3 .
Figure S3.Kinetic data for DOM-eaq -bimolecular rate constant determination for varying ionic strength (IS) (A-B) and pH conditions (C-E).Line represents a linear fit to the data using the least squares method with the slope reported as the bimolecular rate constant ( ,  − ).SRFA II (blue color) and ESHA IV (brown color) utilized as representative DOM isolates.Markers represent pseudo-first-order rate constants determined from transient eaq -decay data and error bars represent uncertainty of the fitted data (majority of error bars are within markers).Insets show data plotted on equivalent y-axis to compare between samples.Experiments conducted at pH 7.0 ± 0.1, 22 ± 2 ⁰C, and 10.0 mM dibasic phosphate buffer unless otherwise specified.

Figure S4 .
Figure S4.Kinetic data for DOM-eaq -bimolecular rate constant determination for varying temperature (A-H).Line represents a linear fit to the data using the least squares method with the slope reported as the bimolecular rate constant ( ,  − ).SRFA II (blue color) and ESHA IV (brown color) utilized as representative DOM isolates.Markers represent pseudo-first-order rate constants determined from transient eaq -decay data and error bars represent uncertainty of the fitted data (majority of error bars are within markers).Insets show data plotted on equivalent y-axis to compare between samples.Experiments conducted at pH 7.0 ± 0.1, 22 ± 2 ⁰C, and 10.0 mM dibasic phosphate buffer unless otherwise specified.

Table S1 .
List of chemicals used in this study.

Table S2 .
List of IHSS isolates used in this study.

Table S3 . SUVA254 and spectral slope measured values for DOM isolates.
a Solutions prepared at standard conditions of 20 ± 2 ⁰C, pH 7.0 ± 0.1, and 10.0 mM dibasic phosphate buffer unless otherwise specified.

Table S5 .
Electron accepting capacity (EAC) and number average (Mn) and weight average molecular weight (Mw) values employed in this study.Mn and Mw values fromLi and McKay. 11 a Electron accepting capacity (EAC) values from Aeschbacher et al.,10 b

Table S6 .
9arameters for calculation of total charge density, Q (meq/gHS),9and the result of these calculations for each DOM isolate at the indicated pH.Information not available for IHSS catalog numbers used in pulse radiolysis experiments (see Section 2.1 in the main manuscript).b Calculations conducted at pH 7 unless otherwise specified. a

Table S9 .
DOM-eaq -bimolecular rate constants for varying environmental conditions.Experiments conducted at standard conditions of 22 ± 2 ⁰C, pH 7.0 ± 0.1, and 10.0 mM dibasic phosphate buffer unless otherwise specified.IHSS catalog numbers for SUVA254 values are different than those used for pulse radiolysis, as explained in main manuscript Section 2.1. a